Transducers convert signals from one domain to another and are often used in sensors. One common transducer operating as a sensor that is seen in everyday life is a microphone, which converts, i.e., transduces, sound waves into electrical signals. Another example of a common sensor is a thermometer. Various transducers exist that serve as thermometers by transducing temperature signals into electrical signals.
Microelectromechanical system (MEMS) based transducers include a family of sensors and actuators produced using micromachining techniques. MEMS sensors, such as a MEMS microphone, gather information from the environment by measuring the change of physical state in the transducer and transferring a transduced signal to processing electronics that are connected to the MEMS sensor. MEMS devices may be manufactured using micromachining fabrication techniques similar to those used for integrated circuits.
MEMS devices may be designed to function as, for example, oscillators, resonators, accelerometers, gyroscopes, pressure sensors, microphones, and micro-mirrors. Many MEMS devices use capacitive sensing techniques for transducing the physical phenomenon into electrical signals. In such applications, the capacitance change in the sensor is converted to a voltage signal using interface circuits.
One such capacitive sensing device is a MEMS microphone. A MEMS microphone generally has a deflectable membrane separated by a small distance from a rigid backplate. In response to a sound pressure wave incident on the membrane, it deflects towards or away from the backplate, thereby changing the separation distance between the membrane and backplate. Generally, the membrane and backplate are made out of conductive materials and form “plates” of a capacitor. Thus, as the distance separating the membrane and backplate changes in response to the incident sound wave, the capacitance changes between the “plate” and an electrical signal is generated.
MEMS microphones with this type of parallel plate capacitive structure formed from the deflectable membrane and rigid backplate may include various performance characteristics as a consequence of the parallel plate structure. For example, the rigid backplate is often perforated in order to allow air to pass through the backplate so that the rigid backplate is acoustically transparent. However, in practice, the rigid backplate often is not fully acoustically transparent and generates some amount of acoustic noise. This often leads to a tradeoff between mechanical robustness, such as by including fewer and smaller perforations in the rigid backplate, and acoustic noise reduction, such as by including more and larger perforations in the rigid backplate.
Another characteristic of such parallel plate structures is the phenomenon known as “pull-in.” In order to operate as an acoustic transducer, a bias voltage is applied between the deflectable membrane and the rigid backplate. Because of the voltage applied between the plates, changes in capacitance between the plates, resulting from motion of the deflectable membrane, produce a measurable voltage signal that corresponds to an incident acoustic signal. However, due to the applied bias voltage, as the separation distance between the deflectable membrane and the rigid backplate decreases, an attractive electrostatic force also increases. The attractive electrostatic force is usually balanced by a restoring mechanical spring force in the deflectable membrane, the attractive electrostatic force increases non-linearly as the distance becomes small while the restoring mechanical spring force increases only linearly. The difference in relation to separation distance results in the attractive electrostatic force overcoming the restoring mechanical spring force when the separation distance reaches a certain limit, which causes pull-in or collapse as the deflectable membrane moves all the way to contact the rigid backplate and may result in stiction. The phenomenon of pull-in presents another tradeoff between resistance to pull-in, from increased rigidity of the deflectable membrane or lower bias voltage, and higher sensitivity, from reduced rigidity of the deflectable membrane or increased bias voltage.
As a further example, dual backplate MEMS microphones are used in order to generate differential signals. Dual backplate MEMS microphones include a deflectable membrane, similar to standard parallel plate microphone, and also include both a top backplate and a bottom backplate above and below, respectively, the deflectable membrane. Thus, as the deflectable membrane moves, the capacitance between the deflectable membrane and one of the two backplates increases while the capacitance between the deflectable membrane and the other of the two backplates decreases. Such structures also exhibit the noise characteristics resulting from perforations in the rigid backplates and are susceptible to the phenomenon of pull-in as described hereinabove.